Step 2
To the same skillet, add 2 tablespoons butter and reduce the heat to medium-high. Add onion and cook until translucent, about 3 minutes. Add the mushrooms and garlic and saute until the mushrooms become tender and their juices evaporate, about 12 minutes. Add the marsala and simmer until reduced by half, about 4 minutes. Stir in the mascarpone and mustard. Return the chicken and any accumulated juices to the skillet. Simmer, uncovered, over medium-low heat until the chicken is just cooked through and the sauce thickens slightly, about 2 minutes. Season the sauce, to taste, with salt and pepper and stir in the parsley.